I want to say "My brother and I are twins" in another way. Is it right to say 'My brother is my twin' ?
The sentence "My brother is my twin" is grammatically correct. There is absolutely nothing wrong with the grammar.
The next question is, "Is it readily understood?" It absolutely is. There is no (reasonable) way to interpret this sentence other than to understand that the speaker has a brother, and the speaker and the speaker's brother are twins.
The last question is, "Does it sound right?" In other words, is it semantically awkward? Under certain circumstances (see the following paragraphs), I would suggest that the following phrases are more common, and by extension, would be considered less awkward:
- "We are twins."
- "My brother and I are twins."
- "My twin brother and I ..."
- "I have a twin brother."
What makes this sentence (potentially) awkward is that any twin of yours will implicitly be your sibling. So there is considerable semantic overlap between the terms "brother" and "twin." Thus, whether or not the sentence is awkward depends on how much information the listener already has (or is expected to have):
- If the listener already knows about the existence of a brother, this would be the appropriate way to further indicate that that person is your twin. It would be equivalent of (as valid as) "Fred is my twin."
- If the listener doesn't even know that the speaker has a brother, then the less awkward phrase would be "I have a twin brother."